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11.8. Overriding vs Overloading

Overriding an inherited method means providing a method in a child class with the same method signature (method name and parameter type list) and return type as a method in the parent class. The method in the child class will be called instead of the method in the parent class. In the following example the MeanGreeter inherits the greet method from Greeter, but then overrides it.

public class Greeter { public String greet() { return "Hi"; } public static void main(String[] args) { Greeter g1 = new Greeter(); System.out.println(g1.greet()); Greeter g2 = new MeanGreeter(); System.out.println(g2.greet()); } } class MeanGreeter extends Greeter { public String greet() { return "Go Away"; } }

Note

To override an inherited method, the method in the child class must have the same name, parameter list, and return type (or a subclass of the return type) as the parent method.

Overloading a method is when several methods have the same name but the parameter types, order, or number are different. In the example below the greet(String who) method overloads the greet() method of Greeter. Notice that MeanGreeter inherits this method and it isn’t overriden.

public class Greeter { public String greet() { return "Hi"; } public String greet(String who) { return "Hello " + who; } public static void main(String[] args) { Greeter g1 = new Greeter(); System.out.println(g1.greet("Sam")); Greeter g2 = new MeanGreeter(); System.out.println(g2.greet("Nimish")); } } class MeanGreeter extends Greeter { public String greet() { return "Go Away"; } }

Note

To overload a method the method must have the same name, but the parameter list must be different in some way. It can have a different number of parameters, different types of parameters, and/or a different order for the parameter types. The return type can also be different.

Check your understanding

10-6-3: Which of the following declarations in Student would correctly override the getFood method in Person?

public class Person
{
   private String name = null;

   public Person(String theName)
   {
      name = theName;
   }

   public String getFood()
   {
      return "Hamburger";
   }
}

public class Student extends Person
{
   private int id;
   private static int nextId = 0;

   public Student(String theName)
   {
     super(theName);
     id = nextId;
     nextId++;
   }

   public int getId() {return id;}

   public void setId (int theId)
   {
      this.id = theId;
   }
}

• public void getFood()
• The return type must match the parent method return type.
• public String getFood(int quantity)
• The parameter lists must match (must have the same types in the same order).
• public String getFood()
• The return type and parameter lists must match.

You can step through an example of this in the Java Visualizer by clicking on the following link Override Example.

10-6-4: Which of the following declarations in Person would correctly overload the getFood method in Person?

public class Person
{
   private String name = null;

   public Person(String theName)
   {
      name = theName;
   }

   public String getFood()
   {
      return "Hamburger";
   }
}

public class Student extends Person
{
   private int id;
   private static int nextId = 0;

   public Student(String theName)
   {
     super(theName);
     id = nextId;
     nextId++;
   }

   public int getId() {return id;}
   public void setId (int theId)
   {
      this.id = theId;
   }
}

• public void getFood()
• You can not just change the return type to overload a method.
• public String getFood(int quantity)
• For overloading you must change the parameter list (number, type, or order of parameters).
• public String getFood()
• How is this different from the current declaration for getFood?

You can step through an example of this using the Java Visualizer by clicking on the following link Overload Example.

What happens if you change the main method in the Java Visualizer to create a new Student object instead of a Person object? Does it still print the same thing?

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